1. Field of the Invention
This invention relates to a process for removing basic, metallic, ash-producing residues remaining in the base-assisted production of poly(vinyl alcohol) polymers from poly(vinyl acetate) polymers. More particularly, it relates to a slurry process which utilizes an ion-exchange resin to separate the basic, metallic ash-producing residues before, rather than after, isolation of the poly(vinyl alcohol) polymer.
2. Description of Related Art
Poly(vinyl alcohol) homopolymer and poly(vinyl alcohol) copolymers, (collectively referred to hereinafter as poly(vinyl alcohol) polymers, or by the abbreviation PVA or PVAs) are produced from the corresponding poly(vinyl acetate) homopolymer and poly(vinyl acetate) copolymers (collectively hereinafter referred to as poly(vinyl acetate) polymers or by the abbreviation PVAC or PVACs) by use of acid or base reactions. The reaction may be, but is not necessarily, catalytic in nature. The process may be catalyzed or otherwise brought about by use of either strong acids or strong bases, and is known as hydrolysis (or saponification) or alcoholysis, depending on the particular reaction. Reactions using base are generally preferred due to greater efficiency and reaction rate. Unfortunately, any unremoved remaining base tends to destabilize the PVA after isolation, during subsequent processing.
Bases are difficult to wash out of the isolated PVA completely, since they tend to be quite strongly absorbed. Base instability may be countered by neutralization of any base remaining before PVA isolation, using weak acids. This produces weakly basic salts such as sodium acetate which are somewhat more amenable to washing out. Such salts themselves, while less destabilizing are, however, still somewhat destabilizing, particularly vis-a-vis recent more demanding uses of PVA. In addition, like other metallic residues, they are ash precursors, in that ash results on subsequent thermal processing of PVA derived products. (Ash, as used here, is the oxide, hydroxide or related complex that results from heating metallic residues).
Metallic ash-producing residues can mount to up to several percent in PVA, if not reduced. In transparent PVA derived products, such as butyrated poly(vinyl alcohol), i.e. poly(vinyl butyral), which is used as a windshield interlayer, ash produces haze at levels above about 0.2 weight percent, and even below this level, can affect weatherability. There is therefore a strong incentive to reduce metallic, ash-producing residues to very low levels.
Remaining metallic residues, when they have been neutralized to form salts, may be removed or reduced by extensive washing of the PVA with water or other solvents, as taught in U.S. Pat. No. 2,642,419. However low salt levels are only achieved by the use of large amounts of solvent, which is inefficient.
Ion-exchange resins (IERs) are known in the removal of metal residues from PVA, as disclosed in U.S. Pat. No. 2,940,948. However, their use required use of an aqueous solution of the PVA. Although this allows ready removal of the IER particles by simple filtration, solid PVA, when required, has to be subsequently isolated by various means. This may involve complex or difficult processes such as re-precipitation, or evaporation and comminution of resulting PVA sheet to particles if PVA is required in particulate form.
The above processes of metal residue removal have been applied either to solid PVA particles, i.e. after PVA isolation (usually removing salts, having neutralized the base residues before isolation), or to PVA prepared as, or converted to, an aqueous solution. It would be highly desirable to have a process to remove ash-precursor metal residues before the steps required to isolate solid PVA, i.e. by removing the basic ash-precursor residues immediately after PVA production, even without neutralization, yet also avoiding solutions of PVA.